The present invention relates to a niobium dioxide (NbO2) sintered compact, a sputtering target manufactured from the sintered compact, and a method of producing a NbO2 sintered compact.
In recent years, a resistive random access memory (ReRAM) has attracted attention as an alternative of flash memories. The ReRAM utilizes a large change in electric resistance by voltage application and is known to have a variable resistance layer made of an oxide of a transition metal, such as nickel, titanium, tantalum, or niobium, in particular, an oxide (suboxide) having a composition shifting stoichiometrically (see Patent Literatures 1 to 3). For example, Patent Literature 1 discloses niobium oxide (Nb2O5) as a variable resistance layer provided in an ReRAM.
A thin film of niobium oxide is usually formed by sputtering. For example, Patent Literature 4, although it is an example of niobium pentoxide, discloses a sputtering target of niobium oxide produced by pressure sintering, such as hot pressing, of a niobium oxide powder.
Meanwhile, in order to most simply produce a sintered compact for a target, the composition of raw materials is adjusted to the intended composition of the sintered compact. If a raw material is expensive, even if the method is simple, the cost is increased to cause a disadvantage from the viewpoint of productivity.
Accordingly, it is known to inexpensively produce a sintered compact by reaction sintering (method of simultaneously performing synthesis and sintering) by using an inexpensive substitute for the expensive raw material. However, in production of a NbO2 sintered compact by applying this method to an expensive material, niobium dioxide (NbO2), for producing by reaction sintering of a Nb2O5 powder and a Nb powder, there occurs problems such as remaining of unreacted raw materials and formation of a large number of small pores in the sintered compact. In addition, the production of a large-sized sintered compact involves a problem of an ununiform in-plane density of the sintered compact.
Patent Literature 1: Japanese Patent Laid-Open No. 2011-149091
Patent Literature 2: Japanese Patent Laid-Open No. 2011-71380
Patent Literature 3: Japanese Patent Laid-Open No. 2007-67402
Patent Literature 4: Japanese Patent Laid-Open No. 2002-338354